Static limiting device



July 13, 1937, D. G. MCCAA STATIC LIMITING DEVICE A Filed Dec. 2, 1935 E MDQL- N manwi CAA HMQDQL v. Y) DAII/ENTOIQ l @W ATT F32 Patented July 13, 1937 UNITED STATES PATENT oFFicE STATIC LIMITING DEVICE David G. McCaa, Lancaster, Pa., assigner to Alan N. Mann, Scarsdale, N. Y., as trustee Application December 2, 1935, Serial No. 52,522 10 Claims. (Cl. Z50-20) My invention relates broadly to static limiting Figure V is a graph showing the operating devices for radio receiving systems4 More specifcharacteristic of the static gate of Figure I. icallymy invention relates to a pair of ther- In Figure I, an antenna l and a ground 3 are mionic tubes arranged to sharply limit their outconnected to the input of a preselector, radio fre- `5 put vcurrents to predetermined values irrespecquency amplifier, and'rst detector which are 5 tive of the -input voltage. The present invention represented by the reference numeral 5. A local isan improvement over my copending application oscillator 'I is coupled to the first detector. Since Serial No. 6,015 led February 11th, 1935, entitled these elements, per se, are not part of my inven- Radio circuits for static limitation. tion, and are well known to those skilled in the l Although numerous current limiter devices art, the details are omitted. It is suiiicient to 10 have been proposed for the reduction of the efpoint out that these elements form the preliminfects Aof atmospheric static on radio receivers, ary stages of a superheterodyne receiving system. most'of vthese devices have handicaps which have The incoming signal and static representing prevented their general adaptation to radio telecurrents beat in the first detector with the curl graphic, radio telephonie and television reception. rents from the local oscillator to form currents 15 For example, the prior art limiters may distort, of intermediate frequency. Within a pair of lower selectivity, require excessive operating voltshields 9, are located a pair of thermionic tubes ages, employ complicated apparatus, or may be I3, l5 which form a gate. Through thiS gate iudifricult to adjust. These difficulties are not prestermediate frequency signal representing eurent in the system disclosedv in the aboveentitled rents are passed to an intermediate frequency 2o oopending application. The system disclosed amplier l1. The intermediate frequency ernthereinpis entirely suitable for receivers of mod- Dlier is coupled to a second detector I9 which erete sensitivity v i may be connected to a suitable audio frequency Hevvevev7 1 have djseevered by the use of a, reamplifier 2|. The audio amplier output is conceiver ofgreat sensitivity connected to the out- Heeted to an indicating instrument 23, which 25 put ofsthe static gate of the above mentioned may be a loudspeaker, cathode ray tube or the system, certain improvements. The present emlike- The intermediate frequency amplifier, SeC- bodiment of my invention may be applied to re- 0nd deteCtOr, audio amplier, aud Signal indicaceivers of widely different degrees of sensitivity tOr Complete the SulJerheterOdyrle receiving SyS with a greatly improved signal to static response tem- These elements, Der Se, are Ilot my inVen- 30 rati@ tion and, being well known to those skilled in the One of the objects of my invention is to limit art, require n0 detailed deSCriptioh. the effects of static impulses on a radio receiv- Having described the system as a whole, I

ing System, AAnm-,her Object is t0 apply a, static Shall 110W deSCll'be the detalls Of the Static gate, 351 limiting gate to the input o-f an extremely sensiand the thermiOriiC tubes emplOyed therein. Irl tive receiver. .A further object is to neutralize Figure II, I have ShOWh the electrode arrangethe interelectrode capacities of athermionic tube ment Of a preferred form of thermionic tube. embodied in a static gate. A still further object The envelope and mounting arrangements are is to. neutralize the effects of space charges about rlOt ShOWn- Within arl evacuated envelope and the screen grid of a thermionic tube employed in suitably mounted are a pair 01"' insulated rnem- 40 a static limiter. Additional objects will be apbers 3|, 33. A unipotential cathode 35 is supparent from the accompanying specification, ported between the centers of the insulated drawing, and appended claims. members 3|, 33. The central portion 3l of the My invention may be best understood by a refcathode is made electron emissive. Surrounding 4:5erencev to thev accompanying drawing in which the cathode, and concentrically spaced there- 45 Figure I is a schematic diagram of one embodifrom and from each other, are a irst grid 39, ment of my invention, a control grid 4|, a screen grid 43, and an anode Figure II is a schematic representation of a 45. The several grids are helical windings of preferred form of thermionic tube employed in uniformly spaced turns of suitable wire. The 5,0.A the static gate, Y helically wound grids extend from one insulated Figure Ill is a circuit diagram which is includmember to the other. ed-,for purposes of explanation, l v y I have found a uniform arrangement of ele- Figure IV is a graph showing the grid voltage, ments including a unipotential cathode, whose plate current characteristic of athermionic tube active surface length is shorter than the grid 5 circuitV embodied in-Figure III, and and anode lengths and concentrically arranged 55 therein is the preferred arrangement. This arrangement lends itself to the sharply defined angular cut-off which I shall presently describe. The heater for the cathode is located within the cathode in the conventional manner. Although I prefer the above described form of tube, it should be understood that my invention is not limited in this respect as other types of thermionic tubes may be employed.

For purposes of explanation, Figure III shows the tube of Figure II connected in a circuit. In this circuit a pair of input terminals 5|, 53 are connected respectively to the control grid 55 of a tube 5l and to the cathode 59 through a by-pass condenser 6|. The lower input terminal 53 is also connected to the slider of a potentiometer 03. A biasing battery 65 is connected across the potentiometer. The positive terminal of the battery is connected to cathode and ground. A space charge grid 01 is connected to cathode 59. A heater 09 is energized by a battery 1| or other suitable current source. A screen grid i3 located between control grid and anode 'i5 is biased positively by a battery The battery may be by-passed by a capacitor 19.

The anode 15 is connected through a resistor 8| of relatively high value to the positive terminal of a battery 33. The negative terminal of the battery is grounded. The battery may be by-passed by a capacitor 85. The anode is connected through a coupling capacitor 81 to an output terminal 89. The other output terminal 9| is grounded. The screen biasing battery may be: combined with the anode battery.

The operation of this circuit is not unlike a resistance coupled amplifier with 'one important exception. The anode resistor is chosen with an ohmic resistance of the order of one quarter to one megohm. The potential of the anode battery is of the order of 221/2 volts. The screen grid potential may be from 12 to 221/2 volts. With a control grid bias of 11/2 to 3 volts, the anode current is very,low. The plate current through the anode resistor plotted against the griad bias is represented in Figure IV.

The curve of Figure IV is taken with conventional meters and shows the following characteristic: The plate current is cut off by a sufficient negative bias on the control grid. The plate current gradually rises as the negative bias is reduced. As the grid is made less negative and beginning at a the plate current increases as a linear function with decreasing grid biases. The plate current appears to rise until it reaches the dash line portion of the curve. At the point b an abrupt angular change takes place. Beginning at b the grid may be made less negative and, in fact, very positive without change in plate current.

If the curve of Figure IV is taken with a highly sensitive direct current amplifier Onth'e output or with dynamic operating characteristics., as, for example, a very sensitive radio frequency amplier coupled to the output of the tube circuit, the characteristic is that of the solid line portion. In the solidline curve two angular changes are observed: The first is represented as an abrupt angular change A. The second is a less abrupt 'but nevertheless angular change A5. The effect o-f these double angles is to diminish the utility of the limiting effect of the tube and circuit of Figure III.

The double angular changes A and AS will either distort desired signals, permit a certain amount of undesired signals to pass, or both effects may be present. This double angle effect prevents sharp, clean cut-off action. I have found a small interelectrode capacity coupling between control grid and anode will account for some undesired signal currents which pass around the gate instead of through it. After exactly neutralizing, static currents still appear which are not properly limited by the cut-'off or limiting action of the tube or gate as I prefer to call it.

Although I do not wish to be restricted to any particular theory of operation, certain observations lead me tobelieve that a space charge effect exists around the screen grid electrode which effects the anode current. Even if no potential is applied to the anode, some of the electrons flowing to the screen grid will pass through Y its meshes and form a space charge effect. The presence of this screen grid space charge has a deleterious effecty on the dynamic operating characteristics of the tube because control grid voltages vary the screen grid current and the space charge effect varies with the screen grid current.

Whatever may be the explanation, I have found that the screen grid space charge effect may be neutralized by using means to stabilize the screen grid current or preferably by the method I am about to describe. After neutralizing the interelectrode capacity, and the screen grid space charge effect, I have'been able to employ two tubes in cascade connection which have sharply defined maximum and minimum output current characteristics. represented in Figure V. The signal lrepresent-` ing voltages C just pass through the gate and are faithfully represented asplate current changes D. Static representingvoltages AE,and F, exceed the gate width sharply cut off.

Having described the improvements of the' present invention over the prior art and the theory of operation, I shall now describe the preferred circuits and adjustments required to accomplish the desired results. Y

Referring to Figure'I, the rst tube I3 of the gate is located within a shield 9. Within this same shield is suitably arranged an intermediate frequency transformer whose primary |03 is connected to the output The secondary is shunted by serially connected capacitors comprising -a tuning capacitor |01 and a by-pass capacitor |09 of low reactance. The junction of the tors is grounded. The lower terminal of the secondary is connected to the slider of a biasingA potentiometer A biasing battery ||3 is"con nected -across the potentiometer.

cathode ||5 of tube |3 and to ground.` To avoid repetition it may be here stated that all batteriesv may be by-passed by suitable capacitors. The

upper terminal of the secondary |05 is connected.

|25. The negative terminal ofthis of a battery to the cathode |I5 which battery is connected is grounded.

This characteristic i'sV or opening and Vare of the first detector.'

tuning and by-pass capaci-v 'I'he positiveterminal of the biasing battery is connected tothe- The biasl battery is repre |23 of relatively low value vof the order of 1000 ohmsto the positive terminal' The anode or plate electrode |21 is connected" through a resistor |29 of relativelyhigh ohmic value to the positive terminal of battery" 13|'. The negative terminal of this battery is connected to the cathode H5. I have foundvthat the anode resistor |29 may have an appreciably capacity reactance. Such reactance may have Aan undesirable effect on the dynamic characteristic curve of the tube and circuit. eliminated by shunting the resistor |29 With a resonant circuit. The shunting circuit comprises a blocking capacitor I33 of very 10W reactance which is serially connected to a grounded inductor |35. The inductor |35 is shunted by atuning capacitor |31. The resonant circuit I35,I31 is grounded. 'Ihe blocking capacitor prevents the grounded resonant circuit from short circuiting theanode battery through the plate circuit-resistor. y

Ihey resonant circuit |35, |31 also "serves a second purpose; namely, neutralization of the control grid anode capacity and screen grid space charge effect. The former capacity is neutralized by connecting a small capacitor v|39 from the control grid II1 to an inductor I4I which is coupled to the inductor |35 of the resonant circuit. The latter effect is neutralized by'connectinga small capacitor |43 to the junction of the screen grid and the resistor |23, and to a second inductor |45 mutually coupled to the inductor |35'of the1 resonant circuit.

'I he second tube I5 of thecascade gate net- Workis'located' within a shield'I I. The "circuit elements of the second tube which are ksimilar to the first tube, described immediately above, are given similar reference numbers. Insofar as the second tube and circuit is similar to the iirst tube and circuit the description will be omitted. 'I'he distinguishing points are that the control grid of the second tube I5 is connected to the anode |21 of the first tube. The cathode of the'second tube is connected to the slider of a potentiometer |41. The potentiometer has connected across it a biasing battery |49 whose nega` tive terminal is grounded. The Vslider is connected to ground by a by-pass capacitor I5 I. The output terminals of the second tube are theungrounded terminal |53 of the resonant circuit andV ground |55. These terminals: are connectedl to the intermediate-frequency amplier I1.`

The method of adjusting the neutralizing elements will now be described. IA strong signal is tuned in. The first gate tube |I3 is biased to cut off. If the signal is still detected in the output device 23, it is because of the interelectrode capacities of tube I3. The neutralizing capacitor |39 is varied until the strongest signals produce no indication in the output device. After the rst tube is neutralized, its control grid bias is restored to a normal operative setting. The control grid bias of the second tube I5 is adjusted to cut-off. The absence of neutralization is recognized by signals in the output. The neutralizing capacitor |39 of the second tube I5 is adjusted for zero output. When this adjustment has been m-ade, the bias of the second gate tube is restored to normal.

If the biases of the two tubes have been adjusted to a value, which will just admit through the gate a desired signal, it may be expected that undesired signal impulses of greater strength would be limited. There is marked limitation but I have found that it is not as complete as the theory predicts. The reason for this incomplete limitation is found in the screen grid space charge effect. This effect may be neutralized by adjusting the screen grid neutralizing This effect "is capacitors I 43 of the first and second gate tubes.

In making this neutralizing adjustment both gate tubes are oper-ated normally to just admit a desired signal. If no natural static is present, any severev local eiect, such 'as a spark coil, will provide'suiiicient undesired signals. The neutralizing capacitors I 43 are both adjusted until the noise or vdisturbing signals are minimized. The optimum condition is relatively easily determined. Once the adjustment is made no readjustment or retuning is required.

The gate Width or signal admittance is adjusted by `regulating the control grid biases of the gate tubes.' If 'the gate is made toosmall, the modulation peaks will be cut off. If the gate is too large, disturbing static effects will be admitted. As pointed out above, neutralization of capacity or neutralization' of screen grid space charge effectis not suincient to accomplish the most effective limitation 'of static. Both effects must be eliminated for optimum results. I do not limit the neutralization to the species shown, as grid, plate, or reverse feedback will serve. Likewise any 4suitableform of filtered rectified alternating current may be substituted for the batteries shown in the accompanying drawing. Since other obvious modications Within the scope of my invention will occur to those skilled in the art, it should be understood that I am not limited to the precise arrangements shown.

I claim:

l. A static limiting device comprising a pair of thermionic tubes, each having control grid, cathode, screen grid, and anode electrodes; the i'irst of said tubes having an input circuit including its control grid and cathode, and an output circuit including its anode and cathode; a resistance included in said output circuit; the second of said pair of tubes having an input circuit including its control grid and cathode and said resistance, andi an output circuit including its anode and cathode and a second resistance; means for establishing a grid voltage-plate current statistic characteristic in each of said tubes in which the characteristic graph is represented by an abrupt angular change between a sloping portion and a horizontal portion,'said abrupt change being effected by less than one tenth volt change of grid potential; and means for neutralizing the effect of varying screen grid currents in each of said tubes.

2. A static' limiting device comprising a pair of thermionic tubes, each having concentric control grid, unipotential cathode, screen grid, and anode electrodes; the rst of said tubes having an input circuit including its control grid and cathode, and an output circuit including its anode and cathode; a resistance included in said output circuit; the second of said pair of tubes having an input circuit including its control grid and cathode and said resistance, and an output circuit including its anode and cathode and a second resistance; means for establishing in each of said tubes a static grid voltage-plate current characteristic which is represented by a sloping portion and a horizontal portion which form an abrupt angle which is effected by a grid potential change of substantially less than one tenth volt; and means for neutralizing the effect of varying screen grid currents in each of said tubes.

3. A static limiting device comprising a pair of thermionic tubes, each having control grid, cathode, screen grid, and anode electrodes; the first of said tubes having an input circuit including its control grid and cathode, and an output circuit including its anodeand cathode; a resistance included in said output circuit; the second of said .pair of tubes having an input circuit including its control grid and cathode and said resistance, and an output circuit including its anode and cathodeand a second resistance; means for biasing the electrodes of each of said tubes so that said tubes have a static operating characteristic in which the input voltages of the first oi said tubes plotted against the output currents of the second of said tubes is represented by a constant current portion, a variable current portion, and a second constant current portion of different value than said rst portion, the intersection of said portions being represented by input potential changes of less than one tenth volt; means for neutralizing the effect of varying screen grid currents in each of said tubes; and means for neutralizing capacity coupling between the input and output circuits of each of said tubes.

4. A static limiting device comprising a pair of thermionic tubes, each having concentric control grid, unipotential cathode, screen grid, and anode electrodes; the iirst of said tubes having an input circuit including its control grid and cathode, and an output circuit including its anode and cathode; a resistance included in said output circuit; the second of said pair of tubes having an` input circuit including its control grid and cathode and said resistance, and an output circuit including its anode and cathode and a second resistance; means for biasing the electrodes of each of said tubes so that said tubes have a static operating characteristic in which the input voltages ofthe rst of said tubes plotted against the 'output currents of the second of said tubes is represented by a constant current portion, a variable current portion, and a second constant current portion of different value than said rst portion, `the intersection of said portions being represented by input potential changes of less than one tenth volt; means for neutralizing the effect of varying screen grid currents in each of said tubes; and means for neutralizing the capacity coupling be- ,tween the input and output circuits of each of said tubes.

5. In a device of the character of claim 1 a space charge grid located between cathode and control grid in each oi said tubes and biased to insure uniform emission from said cathodes.

6. In a device of the character of claim 2 a space charge grid located between cathode and control grid in each' of said tubes and biased to insure uniform electron emission from said cathodes. 5 y

7. A static limiting device comprising a pair of screen grid thermionic tubes, each of said tubes having an input circuit and an output circuit including an anode, resistor, and cathode, means connecting the output of one of saidtubes to the input of the other, resonant circuits effectively connected to each of the output circuits of said tubes, and means fo rneutralizing lthe effect of varying space charges about the screen gridsof said tubes comprising capacitors connected to each of said screen grids andto inductors coupled to said resonant circuits. f

8. Ina device of the character described a pair of thermionic screen grid tubes further characterized by having output circuits whose currents are substantially constant over widely varying positive input voltages, means coupling the output of one of said tubes to the input of the other, a resonant circuit in the output circuit of each of said tubes, inductors coupled to said resonant circuits and capacitors connected to z-.said inductors and to the screen grid and input electrodes of said tubes so that the capacity coupling between input randoutput circuits and the screen grid space charge eiects are neutralized in each of said tubes.

9. In a device of the character of claim 8, a space charge grid in each ofsaid tubes biased to insure steady uniform electron emission.

10. A static limiting device comprising a first and second thermionic screen grid tube, each having control grid, cathode, screen grid, and anode electrodes; resistors connected to said anodes and to a source of current; means for biasing said electrodes so that a grid voltage-plate current static characteristic is established in each of said tubes in which the characteristic graphV is represented by an abrupt angular changebetween a sloping line portion and a horizontal line portion, said abrupt change being eected by less than one tenth volt change of grid potential; a connection effectively including the resistor in one of said anode circuits and the control grid and cathode of the other; means for neutralizing the effectof varying screen grid currents on said maximum constant currents; and means for eliminating the capacity coupling between control grid and anode electrodes in each of said tubes.

DAVID G. MCCAA. 

